Fig. 3 shows the relationship between the drying rate and the
moisture content for the samples treated in (a) the hot-air fluidized
bed at 101 kPa, (b) hot-air fluidized bed at 12 kPa, and (c)
superheated-steam fluidized bed at 12 kPa. These plots were constructed
from the fitting to Page's model in Fig. 2. For the sample
subjected to hot-air fluidized bed drying, the drying ratewas higher
at 12 kPa than at 101 kPa. Under reduced pressure, the boiling point
of water is lower than at atmospheric pressure, and evaporation of
water occurs at a lower temperature. Consequently, the temperature
of a material is lower at a reduced pressure than at atmospheric
pressure (as shown in Fig. 4) and the temperature
difference between the material and the fluidized bed (heating
medium), which facilitates heat transfer, is larger at 12 kPa than at
101 kPa.